1. Field of the Invention
A certain aspect of the present invention relates to a pulse tube refrigerator.
2. Description of the Related Art
Pulse tube refrigerators are widely used to cool apparatuses, such as a magnetic resonance imaging (MRI) apparatus, that require a cryogenic environment.
In a pulse tube refrigerator, a refrigerant gas (e.g., helium gas), i.e., a working fluid, compressed by a gas compressor is repeatedly caused to flow into regenerator tubes and pulse tubes and then to flow out of the regenerator tubes and the pulse tubes back into the gas compressor. As a result, “coldness” is generated at cold ends of the regenerator tubes and the pulse tubes. The cold ends are brought into thermal contact with an object to draw heat from the object.
Take, for example, a two-stage pulse tube refrigerator including a first stage regenerator tube, a second stage regenerator tube, a first stage pulse tube, and a second stage pulse tube.
Normally, the first and second stage regenerator tubes are implemented by cylinders containing a cold storage medium and the first and second stage pulse tubes are implemented by hollow cylinders. One end of each cylinder functions as a hot end and the other end of the cylinder functions as a cold end. A first cooling stage is provided at the cold ends of the first stage regenerator tube and the first stage pulse tube, and a second cooling stage is provided at the cold ends of the second stage regenerator tube and the second stage pulse tube. An object to be cooled is brought into contact with the cooling stages. The cold end of the first stage regenerator tube is connected and communicates with the hot end of the second stage regenerator tube.
Typically, heat exchangers are provided at the cold ends of the first and second stage pulse tubes to transfer the “coldness” from the refrigerant gas (i.e., to transfer heat from the heat exchanger to the refrigerant gas).
However, disposing the heat exchangers at the cold ends of the first and second stage pulse tubes increases the total lengths of the first and second stage pulse tubes and thereby increases the total size of the pulse tube refrigerator. For this reason, in some pulse tube refrigerators, a part or all of the heat exchangers are provided at the cold ends of the first and second stage regenerator tubes to reduce the sizes of the pulse tube refrigerators (see, for example, patent document 1).
Assuming that a heat exchanger is provided at the cold end of the first stage regenerator tube, the refrigerant gas flows into the heat exchanger from the first stage pulse tube and from the second stage pulse tube via the second stage regenerator tube, and heat exchange takes place between the heat exchanger and the refrigerant gas.    [Patent document 1] U.S. Pat. No. 6,715,300 B2
When the refrigerant gas is recovered by the gas compressor, the refrigerant gas flows into the first stage regenerator tube from the first stage pulse tube and from the second pulse tube via the second regenerator tube as described above. Here, the heat exchanger provided at the cold end of the first stage regenerator tube is used to transfer (or absorb) the coldness from the refrigerant gas flowing into the first stage regenerator tube.
However, it is expected that the amount of heat (or coldness) exchanged between the heat exchanger and the refrigerant gas flowing into the first stage regenerator tube from the second stage pulse tube via the second stage regenerator tube is very small. This is because a substantial amount of coldness is transferred from the refrigerant gas to the cold storage medium in the second stage regenerator tube before the refrigerant gas passes through the hot end of the second stage regenerator tube. In other words, the cooling capability of the refrigerant gas is reduced to a low level when it reaches the heat exchanger.
Meanwhile, regardless of whether heat exchange occurs between the heat exchanger and the refrigerant gas from the second stage regenerator tube, the pressure of the refrigerant gas drops as long as the refrigerant gas passes through the heat exchanger. In other words, although no substantial heat exchange occurs between the heat exchanger and the refrigerant gas flowing from the second stage regenerator tube into the first stage regenerator tube, the pressure of the refrigerant gas drops “unnecessarily”.
Such pressure loss may decrease the total cooling capability of the pulse tube refrigerator and therefore has to be reduced.